Many solenoid valves have a ferromagnetic casing. Inserted within the casing is a coil. The coil encircles a flux tube that is magnetically connected with the casing. Axially separated from the flux tube is a pole piece which is also magnetically connected with the casing. Axially separating the flux tube from the pole pieces is a flux choke. Slidably mounted within an inner bore of the flux tube and pole piece is an armature. The armature typically abuts a push rod connected with a valve member which is often a spool type valve. A spool type valve is typically surrounded with a hydraulic housing. The hydraulic housing can often be connected with a control pressure inlet and outlet or port. The control pressure port is typically connected with a hydraulic component of a transmission. A supply pressure inlet or port will be connected with the housing connecting the solenoid valve with a source of pressurized fluid. The housing is often connected with an exhaust port to provide a fluid pressure sump. The solenoid valve will typically be utilized to control the control pressure by selectively connecting the control port with the supply port or the exhaust port. Typically an electronic controller provides signaling to the solenoid valve to move the solenoid spool valve from a neutral or preferred position to a position that facilitates the desired pressure within the control pressure.
In many solenoid valve applications, the spool valve will be spring biased to a position of being normally high control pressure or normally low control pressure. In the normally high position, the spool valve is spring biased to connect the supply port with control port. In a normally low type solenoid valve, the spool valve is spring biased to connect the control port with the exhaust and the solenoid valve is actuated against the spring bias to connect the control port with the supply port.
When a solenoid valve as described above is actuated, the current in the coil creates a magnetic flux loop that goes into the casing to the flux tube into the armature (because of the flux choke) then back into the core shunt into the pole piece and then back into the casing. This flux loop induces the armature to move in its bore within the flux tube and pole piece. The movement of the armature via a push rod is transferred to the spool valve to selectively change the connection of the control port with the exhaust port and/or the supply port.
Certain attributes of the properties of solenoid valves can induce unstable performance. A first property of solenoid valves that can cause instability is that the force output of the armature that is placed up on the spool valve varies depending upon the axial position of the spool valve even when the coil amperage is held at a constant. Accordingly, the armature can place more force on the spool valve when the armature is at its more extended position than at its initial position or vice versa. Another problem associated with solenoid actuators is that the spring which typically biases the armature to a non-engaged position places different amounts of force on the armature depending upon the axial position of the armature. When the armature is at its fully extended position, the spring is typically at its maximum energy storage and therefore places more energy against the armature than when the armature initially moves from its rest position. The properties of the force output of the armature being dependent upon axial position and the resistance force of the spring being dependent upon the axial position can sometimes instigate unstable behavior in the solenoid valve. If the armature force output due to armature axial position variations, activation of the solenoid valve can often cause the armature to move beyond its desired position to either overexpose the control port to the supply port or to the exhaust port. This causes the solenoid valve to cause the control pressure to either increase or decrease beyond what is desired. A solenoid controller is typically unaware of this overreaction of the solenoid valve until there is a sufficient time delay that a pressure sensor, cognizant of the control pressure, can signal the controller to lower the activation of the armature. The controller is cognizant of the pressure within the control pressure, but is not cognizant of the axial position of the armature. Accordingly, the controller based upon a formula or a data table will provide a compensating signal to the solenoid valve to cause the armature to reverse from its prior position. This reversal will often be inaccurate due to the variables of solenoid actuator force and spring force due to the axial position of the armature. Again, the controller will cause the solenoid valve to overcompensate and the result will be an undesirable fluctuation in the control pressure from the control pressure that is desired. It is desirable to provide a solenoid valve that can compensate for the variations in armature force output and spring biasing force output that vary with armature positions.